PROJECTSUMMARY/ABSTRACT Currently, no effective treatment for Malignant Peripheral Nerve Sheath Tumors (MPNSTs) exists outside of radical resection, which has poor long-term survival, putting patients with this aggressive spindle cell neoplasm in dire need of an effective treatment. Loss of Neurofibromin 1 (NF1) and subsequent Ras activation are hallmarks of this disease. Proposed therapies targeting Ras and its downstream effectors have failed, so we are exploring targeting upstream activators of Ras, specifically Receptor Tyrosine Kinases (RTKs), which are activated in a positive feedback loop as a result of NF1 loss. Using pharmacologic and genetic screens, we assessed the contribution of all 58 RTKs in MPNST proliferation and survival, identifying the HER receptor HER3 and Insulin-like Growth Factor 1 Receptor (IGF1R) as critical for MPNST progression. Subsequent validation experiments with the broad spectrum HER inhibitor canertinib and an IGF1R inhibitor, picropodophylin (PPP), showed a decrease in proliferation of MPNST cells, with a statistically significant decrease in cell viability with combination treatment. Levels of activated Ras and phosphorylated Erk1/2 were also reduced following combinatorial treatment. While our preliminary data show a decrease in proliferation of MPNST cells and reduced expression of IGF1R initially after HER3 ablation, we observe that in late passage cells, there is a subsequent increase in IGF1R expression and a rescue in proliferation, suggesting compensation and cooperation between the two RTKs. Our preliminary studies also link HER3 to R-Ras family member proteins, specifically linking loss of HER3 with reduced activation of activated R-Ras and R-Ras2. We have explored the role of R-Ras in MPNST migration and invasion, finding a decrease in migration and loss of invasive phenotype after loss of R-Ras family members (via dominant negative vector expression). We propose to rigorously test the hypothesis that HER3 and IGF1R cooperatively promote MPNST progression and modulate migration and invasion through isoform-specific activation of R-Ras proteins and downstream signal transduction pathways of Raf/MEK/ERK and PI3K/AKT; this will be accomplished through 2 aims: first, examining the cooperation of HER3 and IGF1R as activators of Ras isoforms to drive proliferation, migration and invasion, and secondly by testing combinatorial therapy with HER3 and IGF1R inhibitors as a novel approach to effectively inhibit tumor growth when compared to monotherapy with either agent alone. We hope to elucidate the mechanism of RTKs and Ras isoforms on MPNST progression, coupled with identifying a novel combinatorial therapeutic strategy for those suffering from this and other NF1 driven diseases.